The present invention relates in general to integrated circuits and in particular to a method and circuitry for reliably controlling the state of a switch under varying voltage conditions.
Metal-oxide-semiconductor (MOS) transistors are often used to perform switching function in integrated circuits. While the MOS transistor makes a reliable switch in most circuit applications, there are certain circuit applications that place severe operational constraints on the MOS switch. For example, in telecommunication circuitry a transmitter/receiver integrated circuit (IC) may be required to interface with signals having absolute magnitudes much larger than the power supply voltage for the IC. Controlling the on and off state of an MOS switch in the IC that interfaces with the external signal becomes a challenging task.
For example, if a p-channel MOS transistor is used to switch an input signal that may swing .+-.15 volts (or .+-.25 volts), turning off the switch and keeping it off when the internal power supplies run between .+-.5 volts (or .+-.12 volts) is problematic. Charge pump techniques may be used to internally generate high enough voltages to drive the gate of the p-channel switch. However, when the IC is not powered, the charge pump circuitry does not generate the desired voltage and the switch may be turned on. When using multiple p-channel switches that connect the input terminal to various different types of circuits, having switches that may be randomly turned on at the same time may cause damage to the circuitry.
There is therefore a need for reliably controlling the state of a switch transistor that operates on various high voltage conditions.